In both electronic and film cameras, the determination of ambient exposure level is used to select appropriate light-gathering parameters, such as aperture size and shutter time, to properly expose the light-gathering image receptor, whether it is an electronic sensor or a photographic film. Such parameters are controlled by a variety of aperture and shutter mechanisms. One type of aperture mechanism used to limit the amount of light allowed to pass through a camera lens allows fixed steps of control, with each step of control allowing a predetermined percentage of light (compared to full aperture) to pass through the lens and onto the image receptor.
An aperture mechanism with fixed aperture stops is often a multiple-bladed iris with blades that move to block light from entering a fixed aperture opening in the assembly. As shown in the exploded view in FIG. 1, a typical aperture mechanism 8 of this type includes a set of aperture blades 10 supported for movement within a housing 12 between upper and lower separators 14 and 16. An actuator 20 is connected by pins 20a through slots 14a, 16a in the separators 14 and 16 to the blades 10. This aperture assembly is fastened into the housing 12 by a retainer 18. A rack gear 20b on the actuator 20 mates with a pinion gear 22 on a drive motor 24, which in turn is driven by an aperture controller 26 to several fixed positions. In operation, the drive motor 24 rotates the actuator 20 to regulate the opening defined by the position of the blades 10. By closing completely, as shown in FIG. 1, this aperture mechanism can also provide a shuttering function (for which reason it is sometimes referred to as an aperture/shutter mechanism).
As further shown in FIG. 1, a lens 28 directs image light along an optical path 29 through the aperture mechanism 8 toward a photo receptor, in this case a photographic film 30. The film 30 is supported by a transport mechanism 32 such that a section thereof is presented in an image plane 34 of the optical section (the lens 28 plus the aperture mechanism 8). The aperture blades 10 thus regulate the intensity of the light that is allowed to strike the image plane 34. While shown as three separate movable aperture blades 10a, 10b, and 10c, an aperture mechanism may utilize more or fewer blades. For example, some conventional camcorders utilize one blade that moves with respect to a stationary stop. FIG. 2 shows the central parts of the three blades 10a, 10b, and 10c overlap to generate an aperture opening 11.
As further shown in FIG. 2, the blades 10 can move to multiple "stop positions" which allow more or less light to enter the camera. For example, the controller 26 commands the aperture position to change from stop A (FIG. 2A) to stop B (FIG. 2B). This movement enlarges the aperture opening 11 between the blades, while a movement from stop C (FIG. 2C) to stop B (FIG. 2B) closes the aperture opening 11. In each case the final designated aperture used for capture is nominally identical, yet the actual amount of light passing through the opening 11 in each case is different. This is due to backlash in the gears connecting the motor 24 to the blades 10a, 10b, 10c, or to friction in the assembly which acts to prevent the blades from reaching their intended destination, or to lack of friction which causes overshoot from the intended movement direction. Also, aperture openings may deviate from one another because of size and/or alignment differences of individual blades which comprise the aperture assembly.
Film cameras which utilize aperture mechanisms with fixed stop positions normally constrain the tolerance of the aperture assembly in order to limit the exposure errors which occur at capture. This is often difficult as aperture assemblies exhibit a variety of variation due to mechanical size, friction and electrical drive momentum considerations. This also tends to make the final cost of the aperture assembly more expensive due to the elimination of apertures which are outside the tighter specification. Because these cameras do not attempt to correct the individual aperture assembly response, the final capture exposure often varies as much as .+-.1/2 stop.
Digital cameras which use bladed apertures also exhibit capture exposure error due to the error in moving to a specific aperture position. Often this error is quite different depending whether the aperture/shutter mechanism is opening or closing as a result of backlash and friction. Due to this problem, these aperture/shutter mechanisms must usually be produced with tight tolerances to yield acceptable exposure error, which can be quite expensive. Moreover, repetitive camera exposures can oscillate if viewed upon a real-time monitoring device such as a preview liquid crystal device (LCD), causing extra wear on the aperture mechanism and unpleasant viewing feedback to the camera user.
Since the problem presented by variations in aperture blade movement results in exposure variation, the challenge is to anticipate these exposure variations and to compensate for the resultant exposure errors.